Lighting Research Center Lighting Research Center
    Volume 13 Issue 1
July 2015    
ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. compatible ballasts - An abbreviated list of common ballasts that will provide the necessary circuitry for a photosensor to operate correctly. Other ballasts may also be compatible; contact the photosensor manufacturer for details. continuous dimming - Control of a light source's intensity to practically any value within a given operating range. capacitor - A device used in electric circuitry to temporarily store electrical charge in the form of an electrostatic field. In lighting, a capacitor is used to smooth out alternating current from the power supply. time delay range - For motion sensors, the range of time that may be set for the interval between the last detected motion and the turning off of the lamps. lamp - A radiant light source. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) frequency - The number of cycles completed by a periodic wave in a given unit of time. Frequency is commonly reported in cycles per second, or hertz (Hz). electromagnetic interference (EMI) - The interference of unwanted electromagnetic signals with desirable signals. Electromagnetic interference may be transmitted in two ways: radiated through space or conducted by wiring. The Federal Communications Commission (FCC) sets electromagnetic interference limits on radio frequency (RF) lighting devices in FCC Part 18. electronic ballast - A ballast that uses electronic components instead of a magnetic core and coil to operate fluorescent lamps. Electronic ballasts operate lamps at 20 to 60 kHz, which results in reduced flicker and noise and increased efficacy compared with ballasts that operate lamps at 60 Hz. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. dimming ballast - A device that provides the ability to adjust light levels by reducing the lamp current. Most dimming ballasts are electronic. power - The power used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power for a lamp and ballast or driver combination. Power is typically reported in the SI units of watts. photosensor - A device used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. nadir - In the lighting discipline, nadir is the angle pointing directly downward from the luminaire, or 0. Nadir is opposite the zenith. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels. hysteresis - The dependence of the output of a system not only on its current input, but also on its history of past inputs. The electric light level set by a photosensor with hysteresis, for a certain photocell input signal, depends on whether that photocell signal is increasing or decreasing. Hysteresis provides stable operation in switching photosensors but is undesirable in dimming photosensors.

How well do wireless occupancy sensors perform compared with wired occupancy sensors?

NLPIP last published a report on occupancy sensors1 in 1998 (NLPIP 1998). Despite the number of years that has passed since that report, the technologies used in these sensors and their performance have remained unchanged. Therefore, the reader is referred to that report for a more detailed understanding of these devices.

NLPIP used the procedure described in Appendix: Detailed Methodology to investigate the geometry of the sectors that are sensitive to motion and their sensitivity. As noted in the appendix, NLPIP investigated the location of sensitive sectors in a zone from the central axis of the sensor out to 30° away from the axis, even though the sensors are sensitive to motion beyond this zone.

The results of the investigation are shown in Figure 2. NLPIP found that there was little difference between the tested wired and wireless occupancy sensors from Leviton and Lutron. The WattStopper wireless sensor had fewer sensitive sectors than its wired sensor. The sensors primarily detect occupancy when a person moves into or out of a sensitive sector, so the WattStopper wireless sensor has less ability to detect small motion than its wired sensor in portions of the tested zone.

The tested Leviton and Lutron wireless occupancy sensors had lower sensitivity than the wired sensors from the same brand. NLPIP expects this lower sensitivity could result in a smaller coverage area and diminished ability to detect small motion, but because the decrease in sensitivity is relatively small, it is not likely to have a noticeable effect on occupancy detection.

Figure 2. Occupancy sensor results. The color-shaded areas in each illustration show the thermally sensitive sectors if the sensor were mounted on an 8 ft (2.4 m) ceiling over a person sitting at a desk. NLPIP tested to only 30 from nadir; the sensor is able to detect motion across a wider field than shown. For each sensor, the distance is noted at which only 50% of thermal stimulus was detected; the greater this distance, the more sensitive the sensor is, as discussed in Appendix: Detailed Methodology. Figure 2

Dual-technology sensors (employing both ultrasonic and PIR technology in one device) can detect small motion better than PIR alone and do not require a line-of-sight to detect motion. However, ultrasound detection requires more power than PIR detection. Presumably because of the limited energy available in wireless sensors (which rely on batteries and/or photovoltaic, or PV, modules), only PIR detection technology was offered in wireless sensors at the time of this study. The lack of ultrasonic and dual technology motion detection can be a drawback to wireless occupancy sensors at this time, especially for spaces where furniture such as cubicle partitions may block detection. The increased ability to add and reposition wireless occupancy sensors compared with wired sensors may mitigate issues encountered from decreased detection due to the lack of ultrasound technology or decreased sensitivity.


1 This publication refers to these products as occupancy sensors. They can be configured as either vacancy sensors, whereby the lights must be turned on manually but can be turned off automatically, or as occupancy sensors, whereby the lights are turned on and off automatically.

 

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